There has been known a technology in which a variable value that changes depending on a state of a control system is used as a feedback gain in a feedback control so that the approximation of an output value to a set point and/or the stability of the feedback control is improved. For example, Japanese Patent Application Laid-Open No. 2006-161605 discloses a technology of an EGR control apparatus in which the EGR amount of an internal combustion engine is feedback controlled, wherein when the internal combustion engine shifts from a transitional state to a stationary state, a control gain is gradually decreased to thereby stabilize the control of the EGR amount. Japanese Patent Application Laid-Open No. 2006-249962 discloses a technology of an EGR control apparatus in which the EGR amount of an internal combustion engine is feedback controlled, wherein a control gain is changed according to whether the error between a target EGR rate and the actual EGR rate is positive or negative, to thereby stabilize the control of the EGR rate.